In this experiment, we’ll build a parachute out of paper bags and see that it can slow down the fall of objects significantly.

Equipment

  • Scissors
  • Thick sewing thread or embroidery thread
  • Scotch tape
  • 2 large paper clips, or 3-4 small paper clips
  • A plastic (polyethylene) sandwich bag or shopping bag

    Instructions
    The experiment can be performed as shown in the following video:

Explanation
Gravity pulls all objects on Earth, pulling them towards the center of the planet. That’s why when we hold an object up and then release it, it falls.

As the physicist Isaac Newton formulated in his Second Law, when a non-zero net force is applied on an object, it won’t remain at a constant velocity but rather accelerate, meaning change its velocity.

But this description is not exactly precise, because Earth is surrounded by a thick layer of air. Even if we don’t usually notice it, this air applies a resistance force called “drag”, which resists the motion of objects. The same force applies in water, which is why it is so hard to walk inside a pool.

Drag depends on many factors such as the density of air, the surface area of the falling object and its geometric shape, as well as how fast it is moving. The parachute increases drag such that it cancels out gravity, so that falling objects no longer accelerate but fall at a constant, slow speed. This conforms with Newton’s first law, which says that when the net force applied on an object is zero, it will move at a constant speed.

When you jump out of a plane with a parachute, in the beginning you will fall and accelerate – or, in other words, increase speed. But since drag depends on speed (squared, meaning an object moving at twice the speed of another, has four times the drag acting on it than the other), the point of equal forces is quickly reached. When gravity and drag are at equilibrium, the net force on the falling object is zero, so it stops accelerating and falls at the constant, finite speed it has reached already.


US Military Parachute | Image taken from Wikipedia

When you jump out of a plane with a parachute, in the beginning you will fall and accelerate – or, in other words, increase speed. But since drag depends on speed (squared, meaning an object moving at twice the speed of another, has four times the drag acting on it than the other), the point of equal forces is quickly reached. When gravity and drag are at equilibrium, the net force on the falling object is zero, so it stops accelerating and falls at the constant, finite speed it has reached already.
 

Additional Information
Drag also affects fuel usage in cars; the faster you go, the more strongly drag affects the car. The more drag the car faces, the more fuel the car has to burn to maintain speed. Above certain speeds, the faster you go, the quicker you’ll reach your destination, but the less efficient the mileage gets. The optimal speed in terms of mileage is around 80-90 km/h.

In cars, unlike parachutes, we want to reduce drag, because we don’t want to slow the car down. That’s why car manufacturers try to build “aerodynamic” cars, with as little air resistance as possible and smooth edges like an arrow or a bullet.

Newton also discovered a third law, which is important for understanding plane and missile movement. Watch how to build a jet plane from a balloon here.

 

Dr. Avi Saig
Davidson Institute of Science Education
Weizmann Institute of Science

Article translated from Hebrew by Aviv J. Sharon, M.Sc. student at the Weizmann Institute of Science.

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